Paranthropus robustus was originally discovered in Southern Africa in 1938. The development of P. robustus, namely in cranial features, seemed to be aimed in the direction of a "heavy-chewing complex". Because of the definitive traits that are associated with this robust line of australopithecine, anthropologist Robert Broom erected the genus Paranthropus and placed this species into it.
Paranthropus robustus (considered for a time by the scientific community as Australopithecus robustus) is generally dated to have lived between 2.0 and 1.2 million years ago. P. robustus had large sagittal crests, jaws, jaw muscles, and post-canine teeth that were adapted to serve in the dry environment that they lived in.
After Raymond Dart’s discovery of Australopithecus africanus, Broom had been in favour of Dart's claims about Australopithecus africanus being an ancestor of Homo sapiens. Broom was a Scottish doctor then working in South Africa who began making his own excavation in Southern Africa to find more specimens, which Dart had found earlier. In 1938, at 70 years old, Broom, excavating at Kromdraai, South Africa discovered pieces of a skull and teeth which resembled Dart's Australopithecus africanus find, but the skull had some "robust" characteristics. The fossils included parts of a skull and teeth; all dated to 2 million years old. Fossil sites found on Paranthropus robustus are found only in South Africa in Kromdraai, Swartkrans, Drimolen, Gondolin and Coopers. In the cave at Swartkrans, the remains of 130 individuals were discovered. The study made on the dentition of the hominins revealed that the average P. robustus rarely lived past 17 years of age.
Paranthropus robustus became the first "robust" species of hominid ever uncovered well before P. boisei and P. aethiopicus. Broom's first discovery of P. robustus had been the first discovery of a robust australopithecine and the second australopithecine after Australopithecus africanus, which Dart discovered. Broom's work on the australopithecines showed that the evolution trail leading to Homo sapiens was not just a straight line, but was one of rich diversity.
Paranthropus boisei (originally called Zinjanthropus boisei and then Australopithecus boisei until recently) was an early hominin and described as the largest of the Paranthropus species. It lived from about 2.6 until about 1.2 million years ago during the Pliocene and Pleistocene epochs in Eastern Africa.
The brain volume is quite small, about 500 and 550 cm³, not much larger in comparison to Australopithecus afarensis and Australopithecus africanus. It had a skull highly specialized for heavy chewing and several traits seen in modern day gorillas. P. boisei inhabited savannah woodland territories. Males weighed 68 kg (150 lb) and stood 1.3 m (4 ft 3 in) tall, while females weighed 45 kg (99 lb) and stood 1.05 m (3 ft 5 in) tall. The average adult males were much larger than females (sexual dimorphism), as was the case in virtually all australopithecine species. The back molar teeth were relatively large, with an area over twice as great as is found in modern humans. The species is sometimes referred to as “Nutcracker Man” because it has the biggest, flattest cheek teeth and the thickest enamel of any known hominin.
Some argue that the craniodental morphology of this taxon (e.g., large postcanine dentition, thick enamel, robust mandibles, sagittal cresting, flaring zygomatic region) are indicative of a diet of hard or tough foods such as ground tubers, nuts and seeds. However, research on the molar microwear of P. boisei found a microwear pattern very different than that observed for P. robustus in South Africa which is thought to have fed on hard foods as a "fallback resource. This work suggests that hard foods were an infrequent part of its diet. The carbon isotope ratios of P. boisei suggest that it had a diet dominated by C4 vegetation.